Same-frequency network deployment is adopted in a Long Term Evolution-Advanced (LTE-A) system to greatly improve the utilization ratio of spectrums while causing serious attenuation of a user signal at a cell edge and also suffering from high interference from another cell, and experience of an edge user will be seriously degraded if the issues of signal attenuation and interference are not handled. The technology of Coordinated Multiple-Point (CoMP) can improve the quality of a signal and also lower inter-cell interference by introducing information interaction and joint transmission between a plurality of cells to thereby greatly improve the performance of data transmission for the cell edge user.
Existing transmission schemes of coordinated multi-point can fall roughly into three categories including Dynamic Point Selection (DPS), Coordinated Beam-Forming (CBF) and Joint Transmission (JT) or can be a hybrid scheme of these three transmission schemes. For example the DPS and the CBF can be combined to select a plurality of points dynamically for coordinated beam-forming. A “point” can be defined as a set of geographically co-located transmit antennas, and different sectors with the same site location correspond to different points.
The system configures different Channel State Information-Reference Signal (CSI-RS) resources so that a User Equipment (UE) can measure downlink channels of respective “points”, that is, estimate the downlink channels of the different “points” with use of different CSI-RSs, where a “CSI-RS resource” refers to a combination of a “resource configuration” and a “sub-frame configuration”, both of which are configured by higher-layer signaling. It should be noted that the “point” may not be a physical point but may be a virtual point, and each virtual point corresponds to a CSI-RS resource and is consisted of one or more physical points, or a physical point includes a plurality of virtual points.
Channel State Information (CSI) needed for the transmission scheme is fed back after the downlink channels are measured. The various transmission schemes need to be supported by corresponding channel state information. For example, the DPS needs channel state information of a plurality of points, or channel state information of a specific point and CSI-RS index information corresponding to the specific transmission point. The CBF needs channel state information of a transmission point without a limited Rank Indication (RI) and channel state information of other points at the RI=1 in a coordination set. The JT needs channel state information of respective transmission points and relative phase information of the respective transmission points, or joint channel state information of a plurality of transmission points. The “coordination set” is defined as points participating directly and/or indirectly in transmission of data to a user, and a “transmission point” is defined as a point transmitting data directly to the UE.
An aggregated feedback of a plurality of CSI-RS resources can be made in practice, that is, CSI of a joint channel of the plurality of CSI-RS resources can be fed back, wherein the CSI can include an RI, a Pre-coding Matrix Indicator (PMI) and a Channel Quality Indicator (CQI).
However a Physical Downlink Shared Channel (PDSCH) transmission scheme assumed for CSI in the CoMP transmission mode may be defined based upon a UE-Specific Reference Signal (UERS), and the CSI is measured based on a CSI-RS. Consequently it is impossible at present to apply a measurement result based on a CSI-RS to the transmission scheme based on physical downlink shared channel, due to that no feedback in the CoMP transmission mode can be made.